Fix manifest (java-native-access#120)

Motivation:

The manifest did contain stuff that is not used at all.

Modifications:

Remove unused entries

Result:

Cleanup

pom.xml

@@ -394,10 +394,6 @@
             </supportedProjectTypes>
             <instructions>
               <Export-Package>${project.groupId}.*</Export-Package>
-              <!-- enforce JVM vendor package as optional -->
-              <Import-Package>sun.misc.*;resolution:=optional,sun.nio.ch;resolution:=optional,sun.security.*;resolution:=optional</Import-Package>
-              <!-- override "internal" private package convention -->
-              <Private-Package>!*</Private-Package>
             </instructions>
           </configuration>
         </execution>